Sulfur dioxide is a component in many commercial effluents such as flue gas produced from burning sulfur-containing hydrocarbons such as natural gas, oil and coal. Flue gases are typically emitted from coal-fired electric power plants, boilers and furnaces. Sulfur dioxide is also a component of the gaseous effluents from sulfuric acid plants, sulfur plants, smelters, as well as other chemical and petroleum processes. Sulfur dioxide is an undesirable pollutant in the atmosphere. Accordingly, a variety of processes have been proposed for the removal of sulfur dioxide from gas streams.
Wet flue gas desulfurization processes using lime or limestone are among the most widely used processes to absorb sulfur dioxide from gas streams. In these wet flue gas desulfurization processes, the gas stream is contacted with an aqueous slurry of lime or limestone. The sulfur dioxide in the gas reacts with the slurry to form calcium sulfite or calcium sulfate, or both. These compounds are collected and removed from the processes and the purified gas is discharged to the atmosphere.
Improvements have been made to the basic processes in order to enhance the rate of absorption of sulfur dioxide by the slurry. In general, the improvements have been accomplished by the incorporation of chemical additives to the slurry. Several of the improved lime/limestone slurry absorption processes are described below.
U.S Pat. No. 3,632,306, issued to Villiers-Fischer, et al., discloses a process wherein waste gases containing sulfur dioxide are scrubbed with an aqueous slurry containing solid reactant particles of an oxide or carbonate of calcium, magnesium or barium, together with an additive containing an acidic radical which solubilizes the calcium, magnesium or barium ion so that the slurry absorbs sulfur dioxide from the gas. At col. 3, lines 25 to 27, the patentees disclose that the acid radical consists of an acid intermediate in strength between carbonic acid and sulfuric acid, or a comparable acid salt.
U.S. Pat. No. 3,883,639, issued to Cronkright, Jr., et al., discloses a process which involves contacting a waste gas containing an oxide of sulfur with an aqueous medium having a PH below 7.1 in an autoregenerative cyclic process using limestone in combination with carbon dioxide and a soluble sulfate of a cation, such as potassium, lithium, sodium, magnesium or ammonium or mixtures of these compounds, to remove the oxides of sulfur.
Rochelle and King, in an article captioned The Effect of Additives on Mass Transfer in CaCO.sub.3 or CaO Slurry Scrubbing of SO.sub.2 From Waste Gases, Ind. Eng. Chem., Fundam., Volume 16, No. 1, 1977, 67-75, disclose that both alkali additives and organic acid additives can be used in lime or limestone slurries in order to improve the sulfur dioxide mass transfer. The authors specifically disclose, on page 71, that the most important potential soluble alkali additives are magnesium oxide, sodium carbonate and ammonia. In addition, in Table III on page 72, acetic acid, benzoic acid, glycolic acid, adipic acid, isophthalic acid and sulfopropionic acid are specifically disclosed as organic acid additives.
U.S. Pat. No. 4,409,192, issued to Lichtner, et al., discloses a method of removing sulfur dioxide from flue gases derived from furnaces burning high sulfur content fuels. The method comprises spraying flue gases with an aqueous medium containing (i) a material either dissolved or slurried therein, which material reacts with the sulfur dioxide in the flue gases to produce calcium sulfate and calcium sulfite, and (ii) a composition comprising a substantially linear water-soluble sulfonated polystyrene and an organophosphonic acid derivative having the structural formula (X).sub.2 N--R--N(X).sub.2, where R is a lower alkyl of from 1 to 7 carbon atoms, X is hydrogen or the group --CH.sub.2 PO.sub.3 M.sub.2, where M is a water-soluble cation, the compound being further characterized in that at least one but preferably all X's are --CH.sub.2 PO.sub.3 M.sub.2.
U.S. Pat. No. 4,423,018, issued to Lester, Jr., et al., discloses a by-product stream from the production of adipic acid from cyclohexane containing glutaric acid, succinic acid and adipic acid, which is employed as a buffer in lime or limestone flue gas scrubbing for the removal of sulfur dioxide from combustion gases. The additive disclosed in U.S. Pat. No. 4,423,018 is often referred to as a dibasic acid, i.e., DBA, and is widely used in commercial processes.
Lee and Rochelle, in an article captioned Oxidative Degradation of Organic Acids Conjugated with Sulfite Oxidation in Flue Gas Desulfurization, United States Environmental Protection Agency, Research and Development Project Summary, April, 1988, relates to the oxidative degradation of the organic acid additives often used in flue gas desulfurization systems. On page 1, the authors disclose:
In addition to the expected loss of organic acid additive by entrainment of solution in waste solids, loss by chemical degredation and coprecipitation is also observed. Chemical degradation which is conjugated with sulfate oxidation is the most important mechanism of buffer loss under forced oxidation conditions. PA0 Suitable chelating agents are the polyamines such as hexamethylenetetraamine (HMTA), ethylenediamine, diethylenetriamine, triethylenetetraamine, polyaminopoly-carboxylic acids, such as ethylenediamine-tetraacetic acid (EDTA) and N-(2-hydroxy-ethyl)ethylenediamine-N, N', N'-triacetic acid (HEEDTA) and polycarboxylic acids such as citric acid. Suitable metal ions include Fe(II), Zn(II), Ni(II) and Co(II) and Al(III). The quantity of active metal chelate in the solution may range from about 0.01 to 0.7 gm moles per liter, preferably the range is 0.06 to 0.075, about 0.066 gm moles/liter. The scrubber solution typically may contain more than one chelating agent in the concentration described above since it has been found that the addition of a second agent may enhance removal of NO from the flue gas to greater than 95%.
Thus, oxidative degradation is one of the problems associated with the use of organic acid additives in lime/limestone slurry processes.
Improvements have also been made to enhance the absorption of nitrous oxides which may be present in sulfur dioxide-containing gas streams.
For example, U.S. Pat. No. 4,612,175, issued to Harkness, et al., discloses a method of simultaneously removing SO.sub.2 and NO from oxygen-containing flue gases resulting from the combustion of carbonaceous material by contacting the flue gas with an aqueous scrubber solution containing an aqueous sulfur dioxide sorbent and an active metal chelating agent which promotes a reaction between dissolved SO.sub.2 and dissolved NO to form hydroxylamine N-sulfonates. The hydroxylamine sulfonates are then separated from the scrubber solution which is recycled. At column 2, lines 13-18, the patentees disclose that the active metal chelate is prepared by mixing a soluble metal salt and a chelating agent in an oxygen-free environment for a period of time sufficient for all the possible coordination sites on the chelating agent to coordinate with the metal ion. At column 4, lines 6 to 21, the patentees disclose:
In addition to the oxidative degradation problem, other problems which occur with the use of additives include losses due to high volatility of the additive and low water solubility of the additive, which can limit the amount of additive that can be incorporated in the slurry.
Accordingly, although the processes described above have been useful for removing sulfur dioxide from gas streams, new lime/limestone absorption processes are desired which utilize additives that are less prone to oxidative degradation and have lower volatility and increased solubility.